the best time is in the spring and fall, but the overall best time is usually in late summer/early fall (Sept-Oct). Many people do not realize this, but you can aerate your lawn any time during the growing season.

I like doing most of my aeration in the fall. you can aerate in the spring but alot of times it too wet, and also it opens up the lawn for weeds and crabgrass to germinate so fall would be the best time.

When to Aerate...Spring and fall are both good but for diff reasons. Both are good for the reasons we aerate...Compaction relief, Venting the soil, thatch control and etc.

Spring is usually best for your new customers. Mow the lawn extra short, Aerate, water and fert at the same time and the grass will react quickly with nice green growth. The customer will be pleased with the money spend due to the quick green on the lawn. After the customer understands the benefits of aeration move them to the fall.
Fall aeration gives you all the same benefits plus it is a big plus to overseeding...BUT there usually is not quick response by the grass plants. So a new customer may wonder why they spent the money to have these little dog turds all over the yard when the grass just keeps getting brown.